VHF Direction Finding and Q-Code Bearings

VHF direction finding (VDF) turns the usual arrangement of radio navigation round. With an NDB or a VOR, the ground station transmits and the aircraft works out its bearing. With VDF, the aircraft transmits on its ordinary VHF communication radio and a ground station measures the direction from which the signal arrives, then tells the pilot the result by voice. The aircraft needs no navigation equipment at all, which is why VDF has long served as a get-you-home aid for lost, uncertain or partly equipped pilots.

The bearings are passed as Q-codes, three-letter abbreviations that fix both the direction (to or from the station) and the datum (magnetic or true). The same codes describe ADF, VOR and RMI bearings, so they appear throughout radio navigation exams, from the PPL to the ATPL. VDF services still appear in AIPs: Toulouse-Blagnac, for example, lists a VDF service with the call sign TOULOUSE Homer (TOULOUSE Gonio in French) on six VHF frequencies.

VDF principle and equipment

A VDF station combines a direction-finding antenna system with a receiver tuned to one or more of the frequencies used by the air traffic service. When an aircraft transmits, the equipment senses the direction from which the signal arrives and displays the bearing to the controller or operator, who passes it back on the same frequency by voice (A3E, amplitude-modulated telephony). Three consequences follow:

• the station can measure a bearing only while the aircraft is transmitting;
• VDF gives direction only, never distance;
• the bearing is measured at the station, so it is converted between true and magnetic with the variation at the station, as for a VOR radial.

A direction-finding station is identified in radiotelephony by the call sign suffix HOMER, and it is listed in AIPs with the abbreviation VDF. Its UHF equivalent, used by the military, is UDF.

Being VHF, VDF is limited to line of sight. The maximum theoretical range is 1.23 × (√h₁ + √h₂) NM, with the station aerial and aircraft heights in feet: a station aerial at 196 ft and an aircraft at 2,500 ft give about 79 NM. High ground between the two cuts the range, and so can the aircraft's own attitude, when banking masks its antenna. A pilot low in a valley may be out of reach of a station that answers easily after a climb (see radio wave propagation).

Requesting and using a VDF bearing

The pilot calls the station on its frequency, asks for the bearing wanted, usually a QDM, and transmits long enough for it to be measured. The station replies with the bearing and its accuracy class, for example "QDM 245, Class B".

VDF bearings are used for:

• homing: the pilot flies the QDM as a heading, asks for another, and corrects repeatedly until overhead. A QDM takes no account of wind, so in a crosswind the path curves downwind of the direct line unless the pilot applies a drift correction (see radio navigation aids and position fixing);
• position lines: a QTE is drawn on the chart from the station, and crossed with a second bearing or another aid to give a fix;
• track checks, confirming that the aircraft is where the pilot believes it to be;
• let-downs: some aerodromes publish VDF approach procedures.

Radio waves follow great circles, so a VDF bearing plotted over a long distance on a Mercator chart needs a conversion angle correction, applied at the station. A station at 30°N 010°W that measures the great-circle bearing of an aircraft at 30°N 030°W as 275° has a convergency of 20° × sin 30° = 10° and a conversion angle of 5°, so the line is drawn on the Mercator chart at 270°.

Auto-triangulation

When several VDF stations take a bearing on the same transmission at the same time, their bearings cross at the aircraft's position. Auto-triangulation combines them automatically into a fix, without the pilot doing anything but transmit. It is provided on the international VHF emergency frequency, 121.5 MHz, where it helps locate aircraft that are lost or in distress. France notifies ICAO that, in addition to its other uses, 121.5 MHz may be used there for direction finding. The military equivalent on UHF uses 243 MHz.

The 121.5 MHz signal of an emergency locator transmitter also serves direction finding. The beacon's digital 406 MHz burst is detected by the Cospas-Sarsat satellites, which have not processed 121.5 MHz alerts since 2009; the low-power 121.5 MHz carrier now serves searchers homing on the beacon in the final stage of a search.

Bearing classes A to D

Every VDF bearing is passed with a bearing classification so that the pilot knows how much weight to give it.

A Class A QDM of 270 at 30 NM means, by the 1-in-60 rule, that the aircraft is within 1 NM of the 090 QDR; a Class C bearing at the same range leaves a band 5 NM either side. A position line or homing based on a Class D bearing should be confirmed by other means.

The Q-code system

The Q code is a set of three-letter abbreviations beginning with Q, each standing for a defined phrase. Aviation keeps a handful: the altimeter settings QNH and QFE (see altimeter settings), the runway direction QFU, and the four Q-code bearings. Common abbreviations such as these are spoken as letters, without the phonetic alphabet.

The four bearings differ in two ways only: whether they are measured to or from the station, and whether from magnetic or true north.

View full schematic

QDM and QDR

QDM is the magnetic bearing from the aircraft to the station. The French AIP's list of abbreviations gives it as "magnetic heading": in nil wind it is the heading to steer to reach the station, which is why a pilot asking for a "steer" asks for a QDM. QDR is the magnetic bearing of the aircraft from the station, the reciprocal of the QDM. It is the VDF or NDB equivalent of a VOR radial.

The same codes describe airborne bearings. On an RMI the head of the needle shows the QDM and the tail the QDR. On a fixed-card ADF, the QDM is magnetic heading plus relative bearing, less 360° if the sum exceeds 360°: heading 300°(M) with a relative bearing of 136° gives a QDM of 076° and a QDR of 256°.

QTE and QUJ

QTE is the true bearing of the aircraft from the station, and QUJ the true bearing from the aircraft to the station; each is the reciprocal of the other. Chart meridians show true north, so a QTE can be plotted directly: draw a line from the station at the QTE, measured against a meridian, with no variation applied. That is why a pilot wanting a position line asks for a QTE, and a pilot wanting a heading home asks for a QDM.

Conversion between the magnetic and true codes uses the variation: easterly variation is added to a magnetic bearing to give true, westerly variation subtracted.

• An RMI tuned to a VOR shows a QDM of 330°, so the QDR is 150°(M). With variation at the VOR of 10°E, the QTE is 160°(T) and the QUJ 340°(T).
• Heading 120°(M) with an ADF relative bearing of 330° gives a QDM of 090°(M). With 8°W variation at the aircraft, the QUJ is 082°(T) and the QTE 262°(T).

QFU and runway direction

QFU is the magnetic orientation of a runway, its magnetic direction in the direction of take-off or landing. The runway designator is the QFU rounded to the nearest ten degrees and divided by ten, as seen from the direction of approach: a runway with a magnetic direction of 094° is runway 09, and parallel runways add L, C or R.

Because the designator is rounded, QFU gives the precise figure. French charts and AIP entries use it freely: runway 29 at Saint-Cyr-l'École is QFU 292°, landing at the Megève altiport is compulsory on QFU 153°, and Toulouse-Blagnac's radio failure procedure tells a pilot in IMC who does not know the runway in use to assume QFU 14. A pilot can compare the QFU with the heading indicator when lined up, as a check that the aircraft is on the intended runway and that the heading is correctly set.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.